Measuring Engine Power

[Dazza]

I have recently had my VX engined Caterham (with MoTeC management system ) Dynoed. It has made an enormous difference to the amount of power and the tractability. The graphs I received show the power and torque curves and measure the power delivered at the back wheels. My question is, how does this number relate to the bhp power figure that Caterham quote for the car when you buy it. Also, what is the amount of power loss from the figure quoted for an engine and if that same engine's power was measured at the back wheels. I guess I am wondering if there are different ways to measure the power which will give you greatly differing numbers.

If Mr. Webb is reading this, I would be interested to know whether the famous K2RUM power of 281bhp was measured at the back wheel, and what was the figure originally before he did his amazing mods.

 

 

[murph7355]

You should have a search on the forum as power measurement has been covered before in some depth.

 

It sounds like you had your power measured on a rolling road rather than a dyno - isn't dyno testing done with the engine out of the car and hooked up to a test rig? Thus giving "gospel" power figures at the flywheel? Could be wrong but thought that was the gambit. Rolling roads are where your car is strapped to some big drums and run up...

 

Power loss betwixt engine and rear wheels will vary depending on lots of factors - basically the individual components in the drive train from clutch to tyres. On a rolling road this is calculated by doing a coast down run. Fancy formulae, a set speed in a high gear, a secret masonic technique and you get a figure. You can then use this against a nuts out power run. I think 15%-20% losses are usual but recollection isn't a strong suit of mine this morning.

 

The archives have absolute definitions of all this stuff.

 

As to how your figures relate to Caterham's, who knows. There will nearly always be differences between claimed and actual bhp. The really important thing is that you noticed a nice difference.

 

Also, the same rolling road can give different figures depending on other factors such as the weather, how sticky your tyres were etc etc. I would guess that many have knobs to twiddle to lessen the variance (the only one I've ever been to does), but it's still worth considering. So small differences (5% ish?) are probably within the resolution of the rolling road...

[edmandsd]

Big subject this one and likely to spark a wide debate no doubt.

 

Points i'd like to make are as follows:

Rolling road power readings vary enormously from being conservative to optimistic to occasionally realistic.

Air temp/humidity can effect the results one day to another although not enormously.

I agree that transmission losses usually represent between 15% and 20% of flywheel power output depending on tyre choice and how many people are sitting on the back of the car to stop the wheels spinning (A difference of 5% can represent a lot of hp when you're quoting flywheel figures). To lose more than this really begs the question of 'Where's it gone ??' I once remember seeing an engine on a rolling road where it was continually turning the starter motor i'e. It hadn't disengaged - This would be one of the rare occasions where power losses would increase significantly and you should have seen the state of the starter motor as a result !

 

A rolling road is good to compare results though i.e. Pick one and keep referring back to it so you can at least get an idea of the % increase in performance - even if the starting point is wrong. They are therefore ideal in terms of optimising set up throughout the rev range on the assumption that the readings are either consistently right or consistently wrong.

 

Really the proof of the pudding comes down to performance.

 

I once met a Caterham owner at Santa Pod who claimed to have a 240hp vx engine. His terminal speed over a quarter mile was only just over 100mph indicating that he's probably got closer to standard 2lt vx power. As a guide 230bhp in an R500 gives circa 115mph terminal speeds.

[murph7355]

If you're near London, Dave Walker's rollers are a good bet (hope he doesn't mind me saying that).

 

*Lots* of 7s of all descriptions have been through his hands and there are graphs and spreadsheets aplenty for comparative purposes.

 

I also believe his RR is pretty accurate (I'll be able to confirm this myself this year with any luck).

[Roland]

A bit confused here, help me out please:

 

Is a Dyno or Rolling Road a measuring device?

Giving you the result of what you have.

How was it possible that after being measured you got more power, and more traction?

 

(think I'll measure my garage again, may be a bit longer now!!) (Sorry for the humour).

Thanks,

Roland

[Roland]

A bit confused here, help me out please:

 

Is a Dyno or Rolling Road a measuring device?

Giving you the result of what you have.

How was it possible that after being measured you got more power, and more traction?

 

(think I'll measure my garage again, may be a bit longer now!!) (Sorry for the humour).

Thanks,

Roland

[Roland]

A bit confused here, help me out please:

 

Is a Dyno or Rolling Road a measuring device?

Giving you the result of what you have.

How was it possible that after being measured you got more power, and more traction?

 

(think I'll measure my garage again, may be a bit longer now!!) (Sorry for the humour).

Thanks,

Roland

[Roland]

A bit confused here, help me out please:

 

Is a Dyno or Rolling Road a measuring device?

Giving you the result of what you have.

How was it possible that after being measured you got more power, and more traction?

 

(think I'll measure my garage again, may be a bit longer now!!) (Sorry for the humour).

Thanks,

Roland

[Roland]

A bit confused here, help me out please:

 

Is a Dyno or Rolling Road a measuring device?

Giving you the result of what you have.

How was it possible that after being measured you got more power, and more traction?

 

(think I'll measure my garage again, may be a bit longer now!!) (Sorry for the humour).

Thanks,

Roland

[EFA]

My 281 bhp was a rolling road figure on my engine in more or less its current state of tune. The figure is an estimated flywheel figure using a roll down measurement.

 

When it was last on a proper dyno it gave 273bhp freshly built.

 

 

 

Fat Arn

Visit the K2 RUM siteid=red>

See the Lotus Seven Club 4 Counties Area Website hereid=green>

 

[edmandsd]

Changing to fully sequential Motec M48 pro system also helped release considerably more power than Lumenition equivalent on my engine. M300 is now available (at a considerably higher price) which has even more gizmos - although the chances of increasing outright performance over the M48 system by the same amount are unlikely.

[edmandsd]

What was your power at the wheels Arnie ?

[EFA]

232bhp IIRC

 

 

I'm not convinced the seqential injection works for increased max power - unless it is comparing grouped setups using a low duty cycle.

 

Running 90%+ duty cycle, I would think the difference is negligable.

 

Fat Arn

Visit the K2 RUM siteid=red>

See the Lotus Seven Club 4 Counties Area Website hereid=green>

 

 

 

Edited by - fat arnie on 25 Apr 2002 13:29:19

[murph7355]

Roland - they're both measuring devices, they just do it in different ways.

 

For actual flywheel bhp the dyno is more accurate - it connects to the engine and measures directly from it.

 

You could argue that for real world use, a rolling road is the one to use as it's the power at the wheels that makes you really go. We all have to have a drive train after all.

 

Ultimately both are comparative measures against various "benchmarks", used largely to justify to ourselves the amount of money we've spent on our engines/cars, or perhaps to prove conclusively that your tallywhacker is bigger than the next man's.

 

With 15% more power at the wheels than my car has at its flywheel Arnie's, for example, will be significantly larger than mine. But then he only uses his in straight lines or for doughnuts which isn't putting your manhood to very good use smile.gif

[edmandsd]

Wasted spark Lumenition system resulted in mid 11 second timing slips @ 120 mph. Motec resulted in high 10's at 128 mph. HTR estimated that the BDX had circa 250bhp with Lumenition set up and 290/300bhp with Motec in order to see this level of improvement over a standing quarter mile. The fuel stand off without the sequential set up was appalling !

 

Edited by - edmandsd on 25 Apr 2002 14:11:24

[Simos]

Roland,

to answer your question, a rolling road (or dyno) is a measuring device which gives an indication of what torque/hp you have.

 

To "rolling road" your car is a colloquial verb which means tune your car while it is on the rollers. Rollers enable the tuner to run the engine up to X revs under a load comparable to what it would be doing if you were thrashing it down the road. Said tuner then looks at the emmissions and timing using the probe he's stuffed up your exhaust and the reader on your ignition and makes alterations either to the map or to the jetting of your carbs. In the latter case it means switching off, replacing the jets, emulsion tubes, choke etc with whatever he deems fit and then running it up again to see the results. After several goes you should find the timing and mixture is giving you better hp than before and the last run will give you your final figure through the revband - the fancy curve.

 

You can also use this to cure flatspots and general missbehaviour.

 

Cheers, Simon.

[Old Septimus]

Why should transmission losses be a percentage of flywheel output? Surely the losses are a fixed figure for a particular gearbox - propshaft - diff combination? For a 7 with a beam back axle aren't they about 12 -15 bhp no matter what the output of the engine is?

[murph7355]

The percentage figure was as a guide only and generally holds true (hence being useful as a guide).

 

I would imagine, however, that there is some physics to describe why chucking more power through a given drive train will produce increasingly greater losses. Until eventually said losses are 100% when it breaks (I wouldn't imagine the %age to increase until this point mind)?

[Blatman]

Rolling Road is a "drive in" place to measure your power by strapping your car down and running it up as if you were driving it on the road. It's already been explained on page 1.

A Dyno (short for Dynamometer, or engine test cell) requires the engine to be a long way from the car, usually deep within the R&D area of a major manufacturer or very specialist engine builders/engineers. The two are only similar in that they produce graphs or plots that tell you how powerful you're engine is. A trip to a dyno would be time consuming and expensive.............

[Peter Carmichael]

I've been trying to hold back, but can't help it.

 

At-the-wheels is b0ll0cksville. A rule of thumb percentage loss is fantasy.

 

At-the-wheels (as being described here) is *not* what gets used in the real world because in the real world you can select gears and your tyres are not being bent over a tight radius roller. Drivetrains do not get less coming out at the one end when you put more in at the other end. Therefore "at the wheels" is b0ll0cks. QED

[Tony Martyr]

I was waiting for Peter to crack but I did not expect him to be so polite about it.

If you really are interested in power measurement of engines I suggest you borrow from your library the book Engine Testing Theory and Practice, by Plint & Martyr ISBN 07506 4021 9. I would have suggested you buy it but it is too expensive and that would be advertising.

[murph7355]

Who said putting more in at one end gives you less at the other then?

 

So on one hand you have a comparative measure which is compromised by the rollers radius etc. And on the other you have a comparative measure compromised by the fact that the engine isn't connected to anything that will make it of use (apart from as a noise generator and heat producer. Which is the more indicative of real world application (even if the difference is tiny and neither mean very much).

 

Does anyone have a shed load of data of dyno power outputs and rolling road outputs so that we can do some statistical analysis on what the actual losses were and hence see if they all fell into a certain band.

 

OK on gears, but if you're comparing your own car with itself in a different engine get up (with no changes elsewhere in the drive train), I think you do have a "real world" comparison. And I'm not listening to you know so there. La la la la laaa.

[Peter Carmichael]

"Less out for more in"

 

Have you ever looked at an at the wheels plot? It peaks earlier in the rev range and then power tails away while we know power at the flywheel is still increasing. How do we know this? datalogging lets us run on the road and measure acceleration. The optimum gear change point is *always* above the flywheel power peak. The flywheel power peak reflects what is going on on the road.

 

Even though it is tantalisingly tempting to believe Andy's proposition, it is fundamentally flawed. The losses that are measured on a rolling road *are* much akin to thw aerodynamic and rolling losses you get on the road - they are a function of wheel speed and are dominated by the cube of the wheel speed. Let's change the semantics. Instead of "at the wheels" let's use the term "after losses".

 

On the road at 120mph, *after losses*, at 7000rpm a 1.6 Supersport has zero power. So therefore it is never worth running the engine at 7000rpm. ACCORDING TO MURPHY's LAW

[murph7355]

Now listen here Carmichael, weren't you the one that got the arse when a couple of people twisted what you were saying recently? teeth.gif

 

I'm going to have to re-read your retort after I've had a few so watch this space for a post at 1am.

 

See you later.

 

Murphy of The Law

 

PS It's a good job I can't make the next Sarf London meet or I would have had to have a big discussion with you and set you straight.

[Peter Carmichael]

I do have a stack of Emerald rolling road data and I checked it today.

 

In a series of runs in the same gear of my own car on the same day, the losses varied by 6bhp (38 vs 32 bhp) at 7000rpm, mostly down to the time since the preceding run (i.e. tyre temperature changing the losses). Throwing in a gear change for a run in sixth, the losses increased by 9bhp at 7000rpm because of the extra roller speed.

 

I also plotted the losses against roller speed and there was variation here too, mostly down to tyre temperature (and load over the back axle) again. 56 bhp losses compared to 43 bhp for two runs passing through 120mph roller speed. These were for two runs taken one year apart, showing a 13bhp difference at the wheels yet the at the flywheel figures lined up all the way up.

 

I also have runs for Robert Grigsby showing 58 bhp loss at 110mph and 5900rpm. Equivalent losses for my car were 34bhp at 110mph and 29bhp at 5900rpm. Robert's engine made 144bhp on that run at the flywheel but only 85bhp "after losses". If anybody has seen Robert on track they will suspect that a few more of those 144 horses are being put to use than the 40% losses would suggest.

 

If the losses were 40% why would we bother...